The invention described herein may be manufactured and used by or for the United States Government for Governmental purposes without the payment of any royalties thereon.
The present invention relates generally to magnetic transducers, and more particularly, to magnetic sensors used with a flux concentrator that modulates the magnetic flux going to the magnetic sensor.
Interest is increasing in the development of miniature magnetic sensors for sensing low-frequency magnetic fields in terrestrial, extraterrestrial, industrial, biomedical, oceanographic, and environmental applications. The trend in magnetic sensor design and development is constantly toward smaller size, lower power consumption, and lower cost for similar or improved performance.
There are several types of magnetometers (magnetic sensors with external instrumentation) currently used. The least expensive and least sensitive devices have resolution of about 10xe2x88x921 Oersted (Oe)/Hzxc2xd and typically are Hall effect devices. These devices work by sensing a voltage change across a conductor or semiconductor placed in a magnetic field. Such devices are insensitive and do not lend themselves to applications requiring greater sensitivity, such as that required in brain scan devices and magnetic anomaly detection devices. Magnetoresistive-type magnetic sensors are suited for low-cost, medium-sensitivity applications and have a resolution of about 10xe2x88x925 Oe/Hzxc2xd. Using spin-dependent tunneling magnetoresistive sensors, one can observe 38% changes in the resistivity in fields of a few Oe, see D. Song, J. Nowak and M. Covington, J. Appl. Phys, 87, 5197 (2000).
More sensitive magnetometers exist, but they are typically limited to applications that can tolerate relatively high power size, weight and cost. The most common of these devices are flux gate magnetometers that have a resolution of approximately 10xe2x88x926 Oe/Hzxc2xd and SQUID (superconducting quantum interference device) magnetometers that have a resolution of about 10xe2x88x9210 Oe/Hzxc2xd. Flux gate magnetometers use a magnetic core surrounded by an electromagnetic coil, and are difficult to microfabricate. Additionally, flux gate magnetometers require relatively large amount of power and accordingly do no lend themselves to low-cost, compact, portable design. Though SQUID magnetometers are the most sensitive magnetometers, the apparatus must include a means for cooling to cryogenic temperatures. This makes SQUID magnetometers extremely bulky and expensive to operate. Their size limits their utility because the active superconducting element cannot be placed directly adjacent to the source of the magnetic field, for example the brain. Accordingly, there is need for small, inexpensive, low power magnetometers that have sufficient sensitivity to be useful for a variety of magnetometer applications at low frequencies.
Magnetic sensors used to detect objects that move slowly typically exhibit considerable low-frequency 1/f-type noise (where f is frequency of operation of the magnetic sensor), an unwanted condition. In general, there is a tendency for such devices that have higher sensitivity to also exhibit higher 1/f-type noise. This generally occurs when using magnetoresistive-type magnetic sensors, see van de veerdonk et al. J. Appl. Phys 82, 6152 (1997).
A well-known way of increasing the sensed magnetic field by a magnetic sensor is by use of a flux concentrator, which can enhance a sensed magnetic field by as much as a factor of 50, see N. Smith et al., IEEE Trans. Magn. 33, p. 3358 (1997). An example of such a device is taught in U.S. Pat. No. 5,942,895, entitled xe2x80x9cMagnetic field sensor and current and/or energy sensor,xe2x80x9d that use Hall sensors with flux concentrator components. The magnetization of flux concentrators increases in the direction of the field to be measured. This in turn increases the magnetic field flux at the position of the sensor and, thus, increases the output signal from the magnetic sensor.
The magnetization of the flux concentrator can change by domain wall motion or domain rotation. The latter is the preferred mode because it generates less 1/f noise. There are many different ways and materials that can used for the magnetic material. The overall objective is the largest possible increase in the magnetic field at the position of the sensor without increasing the magnetic 1/f noise.
A magnetic sensor (magnetometer) that addresses 1/f-type noise is taught in U.S. Pat. No. 4,864,237. This disclosure teaches of an apparatus for measuring magnetic fields, which change only at extremely low frequency. This apparatus uses a SQUID magnetometer that includes a superconducting flux transformer that inductively couples a detected signal into a d-c SQUID sensor. This magnetometer can optionally include a device for modulating the detected signal in a frequency range characteristic of low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1-kHz. Limitations of this device include need for cryogenic operation, which inherently do not lend themselves to relatively low cost, low power use.
Thus, there is need, for small, low-cost, low-power-consuming magnetic sensors having sensitivities capable of meeting the varied applications listed above for detecting low frequency signals and minimizing the 1/f-type noise.
It is therefore an object of this invention to provide a magnetic sensor with a modulating flux concentrator having sufficient sensitivity for a variety of applications that minimize effects of 1/f-type noise.
It is a further object of this invention to provide such a magnetic sensor with a modulating flux concentrator that is inexpensive to manufacture, has a high sensitivity, yet not having to be concerned with 1/f-type noise associated with that type of sensor. Such devices can preferably be produced by microfabrication MEMS-techniques.
It is a further object of this invention to provide such a magnetic sensor with a flux concentrator that uses relatively little power. An example would be a magnetic sensor with a flux concentrator that can be operated for long periods of time from batteries.
This invention results from the realization that a small and inexpensive yet extremely sensitive magnetic sensor, may be accomplished by oscillating a torsionally suspended flux concentrator or a flux concentrator that can rotate or oscillate about an axis.
The invention improves the sensitivity of magnetic sensors in general that operate at low frequencies by using flux concentrators that modulate an observed sensed, low-frequency, magnetic field, thereby shifting this observed field to higher frequencies where the noise of the sensor is smaller to minimize 1/f-type noise. This is preferably accomplished by providing a torsionally suspended microelectromechanical (MEMS)-type magnetic flux concentrator or a flux concentrator that can rotate or oscillate about some axis in combination with a magnetic sensor, preferably on a common substrate. Such a combined device is used in a magnetometer. Such a device is small, low-cost and has low-power-consumption requirements. The magnetic sensor can be a Hall effect or other type of magnetic sensor. At least one torsionally suspended or free-to-rotate MEMS-type fabricated flux concentrator is used with the magnetic sensor. The torsionally-suspended flux concentrator or a flux concentrator that can rotate or oscillate about some axis at a modulation frequency much greater than an observed lower frequency signal being sensed.
The signal to noise ratio is improved by this modulation technique if the flux concentrator has less 1/f noise than the magnetic sensor. Since flux concentrators are much simpler devices than magnetic sensors, there are fewer constraints in decreasing their 1/f noise. Several ways of making low noise flux concentrators are discussed herein. Thus, it should be possible to satisfy the condition that the 1/f noise is lower in the flux concentrator than in the magnetic sensor.